Noise cancellation

ABSTRACT

A noise cancellation circuit has a first and second input for a first signal having a signal element and a noise element, and a second signal comprising at least a smaller amplitude of the said signal element, a first inverter arrangement for producing an inverted signal output that is an inverted form of one of the first and second signals, a first adder for adding the other signal and the inverted signal to produce an intermediate signal, an intermediate inverter arrangement for inverting the intermediate signal to produce an inverted intermediate signal, and a second adder for adding the other signal, the inverted signal and the inverted intermediate signal to produce an output.

The present invention relates to noise cancellation.

Noise cancellation in the audio or other frequency ranges is commonlybased on the theory of using a first input of the desired signal plusnoise, and a second input of noise alone. One input is phase invertedwith respect to the other, and the two are then added so that the noise(common to both inputs) is cancelled, leaving the desired signal. Thetechniques used in practice are more sophisticated than this because thebasic theory does not take into account other considerations. The inputtransducers (for example microphones in the audio frequency range) fromwhich the input signals are derived also are non-ideal and, thus, imposedistortions on the two inputs, but to different extents. Furthermore,the theory demands that the desired signal be absent from the secondinput or at least attenuated in it.

Previous techniques for improving on the basic approach to noisecancellation include filtering the output signal. However, filteringneglects to take into account the distortion imposed on the noise signalby the process of transducing it into a manageable electrical quantity,and subsequent signal processing.

Another technique is to digitise the analogue signals and apply digitalsignal processing to address the residual noise.

Other noise reduction methods that have been proposed involve the use ofphased arrays of signal pick-ups (for example microphones). These areinflexible and expensive.

In spite of the fact that the basic technique of noise cancellation hasbeen known for many decades, the incomplete noise cancellation due todistortions imposed on the two signals at the first and second inputshas only ever been addressed by such additional techniques of filtering,etc. which are unrelated to any comparison of the signals themselves.The limit on the extent to which noise cancellation can be effectiveresides, at least in part, in the distortion of the transduced signals.For example, microphones used in pairs or other groupings are notidentical and do not produce exactly the same signals for a given input.A matched pair of microphones can be used to minimise this problem, butthey will never be identical and, in any event, will cost more. Anotherproblem is the spacing of the transducers in relation to the source.Microphones will be located at different positions and, therefore, willbe exposed to slightly different noise stimulation.

According to the present invention there is provided a noisecancellation circuit comprising: a first input for a first signal havinga signal element and a noise element; a second input for a second signalcomprising at least a smaller amplitude of the said signal element; afirst inverter arrangement for producing an inverted signal output thatis an inverted form of one of the first and second signals; a firstadder for adding the other signal and the inverted signal to produce anintermediate signal; an intermediate inverter arrangement for invertingthe intermediate signal to produce an inverted intermediate signal; anda second adder for adding the other signal, the inverted signal and theinverted intermediate signal to produce an output.

Also, according to the present invention there is provided a method ofnoise cancellation comprising comparing a first signal having a signalelement and a noise element, with a second signal having at least asmaller amplitude of the said signal element to produce an intermediatesignal, and subtracting the intermediate signal from a comparison of thefirst signal and the second signal to produce an output.

The present invention provides a particularly beneficial effect that isnot intuitive. This is because the circuit of the invention compares thenoise in signals received, as transduced at the second input, withitself further to reduce the noise which is reduced in any event bycomparing the noise at the first and second inputs separately. Theeffect is to cancel noise by comparing similar responses and, thereby,avoid the effects of distortions due to which the cancellationpreviously effected according to known principles was less effective. Bycomparing the first input, the second input and the intermediate signal,the distorting effect at the circuit input is also taken into account.The noise cancelling effect can be optimised for a given applicationaccording to the relative attenuations/amplifications of the signals atthe second adder.

Preferably, transducers are connected to the first and second inputs bywhich signals are received and to which the noise cancelling process isto be applied.

In a particular form of the invention the first inverter arrangement isarranged to invert the second signal from the second input to producethe inverted signal.

The invention is particularly applicable to the audio frequency range,but is not limited to it. The invention applies to any frequency rangeand applications where the effects of distortion caused by the inputshould be taken into account.

Preferably, the second input can be derived by using transducers inwhich the transducer connected with the first input is constructedand/or arranged to reduce the reception of the signal element by thesecond transducer. For example, when the transducers are microphones,one microphone connected with the second input can be arranged to bebaffled in its reception of the signal element, by the presence of themicrophone connected with the first input in the direction of receptionof the signal elements or by an additional baffle. In the case ofmicrophones, it is found that the receiving faces are preferably spacedby a distance in the range of 0.2 mm to 2.5 mm, preferably 0.625 mm.Alternatively, the baffle can be introduced to attenuate the signalreaching the noise microphone. The attenuation of the signal elementreceived by the second (noise) microphone is due to the baffling effectof the microphone in front, and also the distance of the secondmicrophone from the signal source. Thus, it is preferable that themicrophones are directional.

The subtraction and inversion of signals is preferably carried out usingoperational amplifiers in the analogue domain. However, other analoguecircuit techniques could be used to equal effect, such as transistoramplifiers.

In a preferred embodiment of the invention the first and second signalsare each low pass filtered. This signal conditioning is used to smoothout the characteristic spikes of high frequency noise in the signal. Asa result, the signals to be cancelled present a broader (less acute)target of a lower power spectral density. The cancellation technique ismore effective when applied to the low pass filtered signals because thecircuit is less sensitive to phase distortion and timeshifts of eitheror both of the input signals.

The invention is equally applicable to the digital domain in which someunwanted distortion noise is added at the analogue-to-digital conversionstage when analogue signals are converted into digital data. The sameissues of signal processing distortion can be addressed by comparing theoriginally digitised signal plus ambient noise, an inverted form of theoriginally digitised signal with a small amplitude signal element and aninverted form of the intermediate digital signal.

The invention can be put into practice in various ways, some of whichwill now by described by way of example with reference to theaccompanying drawings, in which:

FIG. 1 is a circuit diagram of one embodiment of the present invention;

FIG. 2 is an illustration of the orientation of audio microphones foruse in the present invention;

FIG. 3 is a circuit diagram of an alternative form of the circuit ofFIG. 1;

FIGS. 4 to 7 are graphic illustrations of signals in the circuit of FIG.1;

FIG. 8 is a circuit according to an alternative embodiment:

FIG. 9 is a generalised block diagram according to the invention; and

FIG. 10 is a circuit according to a further alternative embodiment.

Referring to FIG. 1, a noise cancellation circuit for audio frequenciescomprises a first input 10 for an electret voice microphone 12. Themicrophone is a transducer, converting acoustic signals into analogueelectrical signals. The acoustic signals are accompanied by ambientnoise within the dynamic range of the microphone. It is the noise thatmust be cancelled as much as possible to derive a more faithful signalat the output of the circuit. A second input 14 has an electret noisemicrophone 16 connected to it. Both first and second inputs 10 and 14are connected between a negative voltage rail (−) and respective 5 kohmpull-up resistors 18/20 connected to a positive voltage rail (+) in eachcase. Other forms of microphone could be used, such as dynamic(electromagnetic), crystal or carbon, that do not require any powerconnections. Directional microphones are desirable in order to provideat least some selectivity in picking up the signal.

The signal level for each input 10/14 is buffered by a unity gainnon-inverting operational amplifier 22/24. The outputs of the bufferamplifiers 22/24 for the voice/noise microphones 12/14 are labelled aspoints C and D, respectively, in FIG. 1. The non-inverted signal atpoint D from the noise microphone 16 is connected to the invertinginputs of a pair of inverting operational amplifiers 26/28 which arealso connected to a mid-supply voltage reference level 30 by theirrespective non-inverting inputs, to centre their output with respect tothe full supply voltage swing, The operational amplifier 26 is arrangedas an inverting attenuator with a gain of 0.85, providing a signal atpoint E of the inverted attenuated form of the signal at point D. Theoperational amplifier 28 is arranged as an inverting attenuator with again of 0.72 at point E′ of the signal at point D. Other settings (forexample unity gain) for the attenuation of the signals will depend on,for example, the microphones and other circuit components used and thesupply potential. The gains of the op-amps 26 and 28 are preferably thesame or close enough to provide signals of similar amplitude in ordernot to impose undue burdens on the rest of the circuit or undermine thenoise cancelling functionality.

The signals at points C and E are combined at point F through resistorsR11 and R12 so that the voice signal at C is, in effect, added to theinverted attenuated form of the noise signal at E. The effect of theaddition of these two signals at point F should, in theory, realise thecancellation of the common, but antiphase, signals in each (i.e. thenoise) subject to what attenuation of the signal at D is caused by theattenuator 26. However, it is well known that this is not the case inpractice. This is because the signals received at the microphones 12 and16 are subject to different distortions due, for example, to thenon-linearities in the system, and thermal and temporal component drift.Thus, it is not the case that the noise in one line can simply be afaithful but inverted form of the other. While, in the past, thethinking has been to approach the residual noise problem by filteringand other more sophisticated techniques, the invention makes use of theresult of this comparison to reduce the noise by reapplying it to thecircuit.

The reduced noise at point F is buffered by a further unity gainnon-inverting amplifier 32 and amplified by a compensating invertingamplifier 34 with a gain of 1.95. The output of the amplifier 32 is anintermediate signal consisting of an inverted form of the signal atpoint C which is indicated in FIG. 1 as point G. The signal at G isattenuated relative to the signal at the point E.

At point H in the circuit, the signal from the second invertingattenuator 28 at point E′ is added to the signal at point C and theintermediate signal at point G through resistors R9, R13 and R14. Thecombined signal is buffered by a unity gain non-inverting amplifier 36,filtered by an a.c. couple 38 to remove any dc component, and connectedto an output 40.

FIG. 2 illustrates the arrangement of the microphones 12 and 16 alsoaccording to the present invention. While each microphone can pick upsound from more than one direction, it has a predominant direction ofreception and is, to that extent, directional. It will be seen in FIG. 2that the noise microphone 16 is arranged with its receiving face about0.625 mm (¼″) behind the receiving face of the voice microphone. In thisway, the voice microphone is fully exposed to the desired input signal(i.e. speech), but also presents a baffle to the reception of the samedesired signal by the noise microphone so that the desired signal isattenuated at the noise microphone. In this way, the noise microphonereceives a relatively greater proportion of noise signal input than thevoice microphone. The arrangement of the voice and noise microphones maydiffer according to application, type of microphones used, distance fromthe source of the desired signal, etc., and can be derived empiricallyaccording to circumstances.

FIG. 3 shows a modified form of the circuit in FIG. 1. An amplifier 42with adjustable gain is used to perform the functions of the amplifiers26 and 28. Thus, the signals at points E and E′, which are essentiallythe same are now rendered simply at the point E in FIG. 3, and connectedto the resistors R12 and R14 in parallel. It is found that it is easierto balance the signal by using only a single amplifier at this point.

FIG. 4 illustrates the two transduced signals, as read at points C and Dof the circuit of FIG. 1. They each comprise a wanted voice signal (inthis case a basic sinusoid for the sake of illustration) and adistorting noise component superimposed upon the wanted signal. They aresimilar as both are exposed to the same noise sources, but the voicesignal at point D is slightly attenuated by about 0.15 relative to thatat point C due, at least in part, to the baffling effect of the voicemicrophone 12 in front of the noise microphone 16, and/or their relativedistances from the source of the sound, as shown in FIG. 2.

FIG. 5 shows a comparison of the waveforms at points D and E, whichlatter waveform is the attenuated inverted form of the waveform at pointD. It is also equivalent to the waveform at point E′ as well. Thewaveform at point E can be considered as a negative ‘dirty’ form of thatat D because a small random variation has been introduced into thesignal, associated with small physical and electrical differencesbetween the two microphones 12/16 due, for example, to manufacturingtolerances in the circuit components.

FIG. 6 illustrates the signals at point C, F and G. The signal at pointF is the reduced amplitude, reduced noise signal due to the addition ofthe antiphase signals at points C and E, but it still containssignificant noise products due to the dissimilarity in the transducednoise signal components caused by the differently distorting effects ofthe two microphones. The signal at point G is the inverted andattenuated form of the signal at point F which is itself used in thecircuit. The signal at point C is shown for comparison with the signalat point F to illustrate that there is noise reduction albeit with anattenuated voice signal as well. This is an illustration of the point inthe circuit at which the prior art would apply filtering and othertechniques to address the remaining noise.

FIG. 7 shows the signal at point C compared with the signal at point F(as in FIG. 6) and also as compared with the signal at point H after thesignals at point C and E′ have been added to the inverted form of thesignal at point F (i.e. the signal at point G). As a result of applyingthe invention, the signal at point H is seen to contain far less noisethan at point F for a similar output voice signal amplitude. Theinvention addresses the additional distortion in the signal due to thetransducers. The invention provides a technique that does this by addingthe signal at G, which is the noise-reduced inverted and attenuatedsignal at F from the basic difference between the output of the noisemicrophone at point D.

To arrive at balanced settings for the gains of the various op-amps inthe circuit by which noise at the point H is cancelled to an improvedextent, the resistor R8 on the op-amp 34 is made adjustable. By placinga microphone arrangement as shown in FIG. 2 so that its predominantreception direction is at right angles to a source of noise (so thatboth microphones receive equal noise input) the value of R8 is adjusteduntil the noise output at the op-amp 36 is minimised. It is alsopossible to adjust the values of the gains of the other op-amps in thecircuit, such as those of the op-amps 26 and 28 for example, to the sameend. However, R8 is the convenient resistance to choose as it limits thenumber of adjustments that have to be made.

The invention has been described in terms of audio frequencies. However,the invention is equally applicable to other frequency ranges andapplications in which the distorting effect of transducing one signalinto another form imposes different distortions on the signals to becompared for the purposes of noise reduction.

The invention provides improvements in signal to noise that are ofbenefit both objectively and subjectively. Objectively it is found thatthe signal to noise improvements have particular advantages in speechdecoding schemes such as voice recognition software. Subjectively, theclarity of the reproduced sound is particularly useful in telephony andradio and other analogue/digital speech communication systems.

It will be appreciated from the description that the preferredembodiment uses very readily available components such as operationalamplifiers, basic resistors and capacitors and transducers, and can beimplemented on an integrated circuit very easily. The invention isparticularly suited to incorporation into equipment at the manufacturingstage or as additional equipment for existing products, such as in wiredand wireless telephony.

FIG. 8 illustrates an alternative embodiment of the invention in whichlike reference numerals have been used for like parts. In this circuitinput op-amps 50 and 52 for the signal and noise channels, respectively,have gains. The output also has an op-amp amplifier 54 with non-unitygain. It is necessary to boost the output for certain applications. Itis found that it is beneficial to do this at least partly by amplifyingthe inputs, and subjecting any amplification of the noise to the samenoise cancelling by the circuit, and to limit the amplification at theoutput.

To summarise the operations of circuit, the desired signal plus noise atpoint C is combined both with the inverted form of the more noisy signalat point E′ and the inverted and attenuated form of the noise-cancelledsignal at point G produced by comparing the signals at point C and D.This is illustrated in FIG. 9 in block diagram form. Because ofdistortions, the noise is not sufficiently cancelled for manyapplications at point G as there is not complete identity between thesignals at points C and D. The addition of this signal in adjusted formwith the inverted noise signal and the voice signal substantiallyreduces still further the noise at the output 40 according to therelative choice of amplification/attenuation factors of the signals atthe various points.

FIG. 10 illustrates a further embodiment of the invention. In thedrawing, parts corresponding to those in FIGS. 1 and 8 have been givenlike reference numerals. In this embodiment, the signal at the point Dis applied to the input of a second order Chebyshev low pass filter 60with a 4 kHz cut-off frequency. The output of the filter 60 is invertedby a unity gain inverter 62 to provide the equivalent of point Ereferred to previously. Similarly, the signal at point C is low passfilter by a second order Chebyshev filter 64 and inverted by a unitygain inverter 66.

The output of the inverter 62 is applied to the input of an adjustableamplifier 68, having a variable feedback resistor 70, providing anoutput at point E″ that is equivalent to point E in FIG. 3. According tothe adjustment of the feedback resistors 70, the amplifier may beadjusted to act as an attenuator.

The signals from the respective microphones 12/16 are now in the form ofsmooth (high frequency attenuated), inverted and attenuated (in the caseof the noise microphone signal) outputs. These are added at the point F′(equivalent to the point F in FIGS. 1, 2 and 8) after resistors R11 andR12 to provide a high impedance input to the buffer amplifier 32. Theyare also added at the point H′ after resistors R13 and R14 to provide ahigh impedance input to a further adjustable amplifier 72 having avariable feedback resistor 74.

The added signals at point H′ are connected to the output of theinverter 76 at point G′ (equivalent to point G in FIGS. 1, 2 and 8). Theadded signals at point F′ are buffered by the buffer 32 and inverted byan adjustable gain inverter 76 (equivalent to the inverter 34 in FIGS. 1and 8), having a variable resistor 78. Thus, the added signals at pointsF′ and H′ are combined at G′, which is attenuated relative to the signalat point E, as before.

According to this embodiment of the invention, the filtering andantiphasing is carried out on each signal substantially identically butelectrically separate. The filtered and inverted signals are provided ashigh impedance inputs at resistors R11, R12, R13 and R14. One antiphase(inverted) combination of signals takes place at point F′ and the otherseparately at point H′. The antiphase combining at point F′ is used todetermine a ‘compensation signal’ that is added to the other antiphasesignal at point H′. The result is a optimally antiphase combination ofsignals at H′. The signal at H′ is the basic advantageously noisecancelled output. This is then buffered by buffer 72 and further lowpass filtered in a second order Chebyshev filter 80. The output of thefilter is further buffered by buffer 82 before being a.c. coupled at 40,as before, to provide the conditioned noise cancelled output.

To set up the circuit of FIG. 10 for a given situation, microphones andoutput, the following procedure is used.

-   1. Set the variable resistors 70 and 78 at zero.-   2. Set the variable resistor 74 for full amplifier gain.-   3. Connect an oscilloscope to the output 40 and arrange the noise    and voice microphone so that their predominant direction of    reception is orthogonal to the location of the noise.-   4. Adjust the resistor 70 for minimum amplitude output on the    oscilloscope.-   5. Adjust the resistor 78 further to minimise the output on the    oscilloscope.-   6. Reorient the microphones towards the noise source.-   7. Adjust the resistor 78 and 74 to achieve the desired output    amplitude.

It will be appreciated by those of ordinary skill in the art that, aswell as the digital implementation of the invention, it is also possiblefor other components to be used to the same effect for different partsof the circuits disclosed. For example, the filters in the circuits ofFIGS. 1, 8 and 10 could be any suitable active or passive arrangementthat provides the required cut-off frequency and attenuation rates.Examples include Butterworth, Elliptical and Bessel filters, andinfinite and finite impulse response filters in the digital domain. Thepurpose of the filters 26, 28 or 60, 64 for the microphone inputs is toreduce the typically sharp spikes of noise components in a signal causedby high frequency noise in a given spectrum so that they are in a moresmooth (high frequency attenuated) form. It is found that theattenuation of sharp unfiltered spikes of noise is less effective whenthere is less than perfect phase and amplitude comparison. This has theeffect of reducing the sensitivity of the circuit to phase shiftsintroduced by circuit components and/or the microphones. Thissignificantly improves the noise cancellation performance. Because theinvention does not rely on precise matching of components, the increasedtolerance derived from low pass filtering the signals is of particularlybeneficial effect.

It will be apparent from the foregoing that the present invention can berealised in many different ways. The invention is not limited to thosedescribed herein, but only according to the spirit and scope of thefollowing claims.

1. A noise cancellation circuit comprising: a first input for a firstsignal having a signal element and a noise element; a second input for asecond signal comprising at least a smaller amplitude of the said signalelement; a first inverter arrangement for producing an inverted signaloutput that is an inverted form of one of the first and second signals;a first adder for adding the other signal and the inverted signal toproduce an intermediate signal; an intermediate inverter arrangement forinverting the intermediate signal to produce an inverted intermediatesignal; and a second adder for adding the other signal, the invertedsignal and the inverted intermediate signal to produce an output.
 2. Acircuit as claimed in claim 1 in which the first inverter arrangementcomprises a first inverter having an output of a first inverted signalwhich is operably connected with the first adder, and a second inverterhaving an output of a second inverted signal which is operably connectedwith the second adder.
 3. A circuit as claimed in claim 1, includingmeans for balancing the amplitude of the noise in the first signal, theinverted signal and the inverted intermediate signal, such that it issubstantially cancelled in the output.
 4. A circuit as claimed in claim3 in which the means for balancing include at least one variable gainamplifier.
 5. A circuit as claimed in claim 4 in which the firstinverter includes the variable gain amplifier.
 6. A circuit as claimedin claim 4 in which the intermediate inverter includes the variable gainamplifier.
 7. A circuit as claimed in claim 4, in which the output ofthe second adder is connected to the variable gain amplifier.
 8. Acircuit as claimed in claim 1 in which the intermediate inverterattenuates the intermediate signal.
 9. A circuit as claimed in claim 1,including a first transducer operably connected with the first input,and a second transducer operably connected with the second input, thesecond transducer being constructed and/or arranged to receive at leastan attenuated amplitude of the signal element relative to the signalelement received by the first transducer.
 10. A circuit as claimed inclaim 9 in which at least one of the first and second transducers is]constructed and arranged to inhibit the reception of the signal elementby the second transducer.
 11. A circuit as claimed in claim 9 in whichthe transducers are microphones.
 12. A circuit as claimed in claim 9 inwhich the transducers are microphones and in which the second microphoneis arranged with a baffle to reception of the signal element.
 13. Acircuit as claimed in claim 12 in which the microphones are directional,the first microphone being arranged in front of the second microphonealong the major direction of reception of signal thereby.
 14. A circuitas claimed in claim 13 in which the receiving faces of the microphonesare spaced by a distance in the range 0.2 mm to 2.5 mm, preferably 0.625mm.
 15. A circuit as claimed in claim 1 in which the first inverterarrangement is arranged to invert the second signal from the secondinput to produce the inverted signal.
 16. A circuit as claimed in claim1 in which the first and second signals are low pass filtered toattenuate higher frequency noise.
 17. A method of noise reductioncomprising: comparing a first signal having a signal element and a noiseelement and a second signal having at least a smaller amplitude of thesaid signal element to produce an intermediate signal; and subtractingthe intermediate signal from a comparison of the first signal and thesecond signal to produce an output in which the noise is reduced.
 18. Amicrophone arrangement for a noise cancellation circuit, comprising afirst microphone arranged to receive a signal element and a noiseelement, a second microphone arranged with a baffle such that itreceives at least a smaller amplitude of signal element relative to thefirst microphone.
 19. A microphone arrangement as claimed in claim 18 inwhich the second microphone is arranged in relation to the firstmicrophone such that the first microphone acts as a baffle to receipt ofthe signal element by the second microphone.
 20. A circuit as claimed inclaim 9 in which the transducers are microphones and in which the secondmicrophone is arranged with a baffle to reception of the signal elementand in which the second microphone is arranged at a greater distancefrom the signal source than the first microphone.
 21. A circuit asclaimed in claim 9 in which the transducers are microphones and in whichthe second microphone is arranged at a greater distance from the signalsource than the first microphone.